Pressure switches have conventionally been employed as safety apparatus for use in hot water supply equipment. Although a fan for exhaust air rotates when hot water supply equipment works, there is a possibility that incomplete combustion may be caused and carbon monoxide may be produced if the hot water supply equipment works with the fan not rotating. Therefore, in order to detect an air blast from the fan, wind pressure is transmitted as an external force and a pressure switch for performing a contact operation on contacts is used.
When one of two movable pieces receives the external force in a state in which the two movable pieces are connected to each other via a flat spring, the movable piece causes a continuous movement thereof according to the external force. At the moment when the movement of the movable piece which has received the external force reaches a certain position, the other movable piece causes a movement thereof quickly. The prior art pressure switch uses a method of performing a contact operation on the contacts using such a snap action mechanism (for example, refer to patent reference 1). That is, the snap action mechanism functions as a switch which is placed selectively in either of two positions according to the external force.
Hereafter, the prior art pressure switch disclosed in patent reference 1will be explained with reference to FIGS. 10 to 15. FIG. 10 is a diagram showing main constitution parts of the prior art pressure switch. FIG. 10(a) shows individual parts, and FIG. 10(b) shows a top plan view in a state where they are coupled to one another.
A movable piece 16 and a load adjustment plate 18 which are shown in FIG. 10(a) are fixed to each other with caulking at a joined part 18a. A hinge portion 17c of a movable piece 17 and the load adjustment plate 18 are fixed to each other with caulking at a joined part 18b. Furthermore, in the state of FIG. 10(b), the hinge portion 17c of the movable piece 17 is fixed to a base 15 made from a resin with caulking at a joined part 17d thereof (refer to FIG. 11). A flat spring 19 has engaging portions 19a at both ends thereof, the engaging portion at one end being engaged with an engaging portion 17e of the movable piece 17, and the engaging portion at the other end being engaged with an engaging portion 16b of the movable piece 16, and the flat spring 19 is shaped like a character C in a state in which openings are opposite to both the movable pieces 16 and 17, respectively.
FIG. 11 is a diagram showing the operation of the movable piece 16. FIG. 11(a) shows an initial state, and FIG. 11(b) shows a state in which a plunger 4c connected to a diaphragm with the wind pressure comes down so that the movable piece 16 is pressed downwardly. In order to make it easier to understand the operation of the movable piece 16, the flat spring 19 is omitted in the figure.
When the external force from the plunger 4c acts on the movable piece 16, an elastic deformation portion of the movable piece 16 curves and produces a reaction force, and the curving stops at a time when the reaction force has a balance with the external force. This elastic deformation portion serves as a supporting point.
FIG. 12 is a view showing the operation of the movable piece 17 of FIG. 10. Contacts 17a and 17b are attached to both sides of a leading end portion of the movable piece 17, respectively. In FIG. 12(a), the contact 17b is brought into contact with a lower contact 14a, and, In FIG. 12(b), the contact 17a is brought into contact with an upper contact 12a. In order to make it easier to understand the operation of the movable piece 17, the movable piece 16 and the flat spring 19 are omitted in the figures.
Although the movable piece 17 also causes an elastic deformation thereof, the movable piece 17 can be moved only between the upper and lower terminals 12a and 14a. This elastic deformation portion also serves as a supporting point, like that of the movable piece 16.
FIG. 13 is a diagram showing the operation of the prior art pressure switch. FIG. 13(a) shows either an initial state or a returned state, and FIG. 13(b) shows a state in which the movable pieces 16 and 17 are reversed because of the external force.
Because the leading end portions of the movable pieces 16 and 17 are connected to each other via the flat spring 19, a repulsive force (designated by an arrow of FIG. 13) acts between the both leading end portions. In FIG. 13(a), assuming that the leading end portion of the movable piece 16 is oriented upwardly, a repulsive force acts downwardly on the leading end portion of the movable piece 17. Therefore, the contact 17b disposed on the lower side of the leading end portion of the movable piece 17 is brought into contact with the lower contact 14a. On the other hand, the position of the movable piece 16 is restricted by the plunger 4c. 
When the external force is exerted upon the movable piece 16 via the plunger 4c in the state of FIG. 13(a), the elastic deformation portion of the movable piece curves and the rigid body portion moves downwardly. When the rigid body portion of the movable piece 16 further moves with increase in the external force and then reaches a certain point, the moment which presses the contact 17b of the movable piece 17 toward the lower contact 14a is reversed. As a result, the movable piece 17 moves upwardly quickly and the contact 17a is brought into contact with the upper contact 12a. FIG. 13(b) shows this state and this series of operations are referred to as a reversing operation.
Next, when the external force is decreased gradually, the bending of the elastic deformation portion of the movable piece 16 decreases, and the rigid body portion of the movable piece 16 moves upwardly (i.e., it starts returning to its original position). When the rigid body portion of the movable piece then moves to a certain point, the moment which presses the contact 17a of the movable piece 17 toward the upper contact 12a is reversed. As a result, the movable piece 17 moves downwardly quickly and the contact 17b is brought into contact with the lower contact 14a again. Thus, the movable piece returns to its original state (i.e., a returned state) of FIG. 13(a), and this series of operations is referred to as a returning operation.
The switch mechanism using the above-mentioned reversing operation and returning operation is referred to as a snap action mechanism, the operation of the pressure switch is determined by the geometric positions of the movable pieces 16 and 17.
FIG. 14 is a diagram showing a load adjustment mechanism of the prior art pressure switch. FIG. 14(a) shows either an initial state or a returned state, and FIG. 14(b) shows a reversed state. FIG. 15 is a view showing deformation of the hinge portion of the movable piece 17.
The magnitude of the reaction force (load) of the movable piece 16 against the external force can be adjusted. In a case in which the elastic deformation portion of the movable piece 16 is beforehand bent by a certain degree, the reaction force of the movable piece 16 increases. Therefore, even if the movable piece 16 undergoes the same displacement, the above-mentioned reaction force has a balance with a larger external force than that in the case in which the elastic deformation portion is not bent at all in advance. By using this principle, the load which causes the switch to reverse can be adjusted to a desired value.
The load adjustment mechanism will be explained below. The hinge portion 17c of the movable piece 17 is joined to the base 15 at the joined part 17d thereof, as mentioned above (refer to FIG. 10(b)). As shown in FIG. 14, when a leading end portion 18c of the load adjustment plate 18 is pushed upwardly by a setscrew 20, the hinge portion 17c of the movable piece 17 becomes deformed with the joined part 17d serving as a supporting point, and the load adjustment plate 18 is lifted (refer to FIG. 15). Because the joined part 18a at which the load adjustment plate 18 is joined to the movable piece 16 drops simultaneously, bending occurs in the elastic deformation portion of the movable piece 16. Because the distance between the joined part 17d of the movable piece 17 and the leading end portion 18c of the load adjustment plate 18 which is pushed up by the setscrew 20 is short, the hinge portion 17c of the movable piece 17 becomes deformed to have an acute angle until its deformation reaches a plastic zone through an elastic zone, and therefore plastic deformation occurs in the vicinity of the joined part 17d of the hinge portion 17c. Therefore, after that, when the setscrew 20 is loosened, the hinge portion 17c does not return to its initial position. More specifically, the load adjustable range becomes narrow. Although this problem can be solved by lengthening the distance between the joined part 17d of the movable piece 17 and the leading end portion 18c of the load adjustment plate 18, there arises another problem that the longitudinal size of the movable piece 17 increases, and therefore the size of the whole apparatus increases. [Patent reference 1] JP,5-114341,A
The prior art pressure switch has the two independent movable pieces, must measure the load precisely, and must have the mechanism of adjusting the load. A problem with the prior art pressure switch is therefore that the parts equipped in the movable pieces 16 and 17 are combined intricately and the number of the parts is large, while it is necessary to define a positional relationship among the parts and assemble them precisely.
A further problem is that when the hinge portion 17c is made to become deformed once, it does not return to its initial position because the deformation of the hinge portion reaches a plastic deformation zone, and therefore the readjustment of the load becomes difficult.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a pressure switch which does not need precise positioning of two movable pieces in a process of assembling the pressure switch.